1. Field of the Disclosure
The disclosure relates to a fiber pump source configured with a MM seed source emitting a smooth low signal light which is coupled to an Yb MM wavelength converter of emission of semiconductor laser diodes to an amplified, ultra bight and low noise pump output in a wavelength band from about 974 to about 1030 nm.
2. Known Art
Fiber lasers including Yb-doped gain fibers are highly efficient, cost-effective, compact and rugged light generating and light amplifying devices. The Ytterbium-based fiber lasers dominate the industrial fiber laser market mainly due to its excellent efficiency and long term stability.
The highest peak absorption in Yb-doped media occurs at about 974-976 nm depending on dopants which are added to the core material. Accordingly, pumping an Yb-doped media at the wavelength as close as possible to the peak absorption wavelength such as 975 nm, has two distinct advantages over other pump wavelengths: shorter fiber lengths of and higher efficiencies of Yb fiber gain blocks. The former is particularly significant because shorter fiber lengths of the pumped Yb gain block are key to limiting deleterious non-linear effects affecting its power scaling. The non-linear effects and other obstacles explained hereinbelow thus may hinder power scaling of high power Yb fiber laser architectures with high beam quality, i.e., high brightness.
To successfully overcome at least some of these obstacles it is necessary that a pump source for HP Yb gain block be configured so as to emit pump light not only having the highest possible pump power, but also the highest brightness and power density, and lowest possible noise level at the peak absorption wavelength.
A viable solution to pump power increase could be the principle of beam combining, which essentially means combining the outputs of multiple laser diodes in a MM combiner so as to obtain a single output beam directly coupled to a pumped Yb gain block. Pump sources based on this principle are further referred to as a MM combiner pump source. While the power output by MM pump sources can be very high neither brightness nor power density nor noise level typically is not markedly improved, if at all.
To improve the brightness of the pump light its divergence should be decreased or/and the beam size should be decreased all through increasing energy per mode in MM radiation of the pump light. The brightness of multiple laser diodes in the MM combiner pump source is however conserved, not improved since there is no further amplification of the pump light emitted by semiconductor lasers.
The increased number of laser diodes, which are necessary for increasing power of a MM combiner pump output, does not come without certain undesirable consequences. The core diameter of the delivery fiber which guides the combiner's output to a gain block should be enlarged with increased powers, otherwise coupling pump light into the delivery fiber causes prohibitively high power losses. Once the core diameter is increased, even with great pump powers, the power density remains substantially the same as before the power was increased. This leads to, at best, the same pump absorption, i.e., the overlap of pump light and active core, in the pumped Yb gain fiber, (further referred to as gain block), remains substantially the same. Without reducing the length of the Yb-doped active fiber, thresholds for nonlinear effects that limit the gain and beam quality are not raised.
Based on the foregoing, a MM combined pump source offers high and very high pump light powers. However brightness and power density of pump light are not beneficiaries of the increased pump light power. Accordingly, an improved fiber pump source outputting high power light in a 974-1030 nm range with great brightness and high power density is needed so as to provide further power scaling and beam quality in Yb gain blocks.
Alternatively, there is always a possibility to configure a pump source including a high power multimode fiber laser which would be able to generate high power pump light at the desired wavelength. However, undesirable power spikes, which are generated in the resonator because a few exited modes interfere with one another—so called speckling effect, may be powerful enough to destroy the laser. But even if the laser is not damaged, its output fluctuates and has a high level of noise which is obviously undesirable.
Thus, there is a need to provide a fiber pump source outputting stable high power, ultra bright pump light in a 974-1030 nm range and in particular at a 975 nm wavelength.
Another need exists for a high power pump source operative to output high power, stable pump light in a 974-1030 nm range with considerably improved brightness by comparison to the known MM combined pump sources.
There is a further need to provide a high power ultra bright fiber pump source outputting pump light in 974-1030 nm range that is characterized by:
a power density in a pump delivery fiber which is at least 10 times higher than currently available power densities available in most powerful MM combiner pump sources, and
pump light power levels exceeding currently available power levels.
Still another need exists for a high power fiber pump source with a multimode seed source, outputting a smooth, low-noise signal light, and an Yb-doped wavelength converter which is configured to amplify the signal light so as to output a pump light with noise level, which does not exceed the noise level of the light signal, and brightness that greatly exceeds the brightness of the light signal.
Yet a further need exists for a high power, ultra bright pump source configured with a seed source and a wavelength converter which is operative to convert emission of a plurality of high power semiconductor sub-pump laser diodes at a wavelength λsp to a pump signal at a wavelength λp so that Δλ=λp−λsp<0/1λSp.